Programme Contents

Regulation of atrogin-1 and protein degradation following incubation with dexamethasone and TNFα in mouse C2C12 and primary human myotubes

Introduction: Atrogin-1 is a muscle specific E3-ligase involved in muscle wasting (Bodine et al., 2001). Increased levels of atrogin-1 mRNA has been observed in numerous in vitro and in vivo rodents models of muscle atrophy (Glass, 2005). Human studies performed have shown that atrogin-1 is increased in human atrophy conditions, such as leg immobilization (Jones et al., 2004), ALS (Leger et al., 2006), COPD (Doucet et al., 2007) and quadriplegic myopathy (Di Giovanni et al., 2004). In mice, but not humans, fasting increases atrogin-1 (Sandri et al., 2004; Larsen et al., 2006), suggesting that species differences may exists with respect to its regulation. The aim of the present study was to determine the regulation of atrogin-1 and protein degradation following treatment with dexamethasone (DEXA) and TNFα in mouse C2C12 and human primary myotubes.

Methods: Mouse C2C12 myotubes and primary human myotubes were treated with either TNF-α (20 ng/mL) or DEXA (10 μM) for 1, 4, 24 and 48-h. Atrogin-1 mRNA levels were measured using real time-PCR. Protein degradation was determined by measuring the release of [³H]-tyrosine into the media.

Results: Atrogin-1 mRNA was significantly increased 2- and 4-fold in C2C12 myotubes after 24 and 48-h treatment with DEXA, respectively. In human myoblasts atrogin-1 was increased 2.2-fold only after 48-h of DEXA treatment. After treating C2C12 cells with TNF-α, atrogin-1 showed a transient change, increasing by 50% following 1-hr of treatment, decreasing to 50% below control levels following 4h of treatment then retuning to control levels after 24 and 48-h. In contrast, human myotubes treated with TNF-α showed a 3.1 fold increase in atrogin-1 after 48-h of treatment. In the human myotubes the increase in atrogin-1 mRNA levels following 48-h of both DEXA and TNF-α treatment resulted in significant increases in protein degradation by approximately 15%.

Conclusions: Treatment of both mouse C12C12 myotubes and primary human myotubes with DEXA results in increases in atrogin-1 mRNA; however human myotubes require a longer treatment period. Treatment with TNF-α demonstrated a more dramatic species dependent effect, with mouse C2C12 myotubes presenting a rapid increase, then decrease in atrogin-1 over 1-4 h of treatment, followed by a return to baseline lines at 48-h. In contrast, human myotubes had an increase in atrogin-1 mRNA 48- after treatment. In human myotubes the increases in atrogin-1 caused by both DEXA and TNF-α was associated with an increase in protein degradation. These observations highlight the need for caution when translating results obtained in rodent models to human conditions.

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